When a driver backs a motor vehicle, he or she must see an obstacle or the like accurately by his or her own eyes or a back mirror. But in view of the structure of the body of the motor vehicle, it is extremely difficult to see an obstacle which exists within the dead angle. Therefore, there is installed a mirror of a specific structure or an obstacle detecting device using a high frequency wave or an infrared rays. The disadvantage of the mirror of a specific structure is that it is often not available for installation in view of the structure of the motor vehicle body, and that it is difficult to catch an obstacle at the night time or at a dark place or at the time of snowing or raining. On the other hand, the obstacle detecting device made of a transmitter-receiver which aims at confirming existence of an obstacle by detecting the reflection time from the obstacle has such a disadvantage that it is difficult to detect a direction of the obstacle, while it is possible to detect a distance between the obstacle and the motor vehicle. Further, even if the transmitter-receiver adopts a sweeping system for a certain speed rotation, more than twice measurings are required, and it is very important to eliminate false signals. Still further, since it is required to actuate the transmitter-receiver mechanically, a complicated mechanism must be arranged.
For the sake of eliminating the above disadvantages of the conventional art, the present invention has been achieved.